An electron microscope of the prior art is shown in FIG. 1. In this arrangement a beam of electrons is passed down a central passage following the line of arrow 10. This beam is the primary beam that strikes a sample 12. Particles of different types return from the sample and may follow a range of paths. It is to be noted that, throughout the present specification the term "return" is used in relation to particles traveling from a sample broadly in the opposite direction to the primary beam and includes primary particles that strike the sample and bounce off as well as secondary particles that are emitted from the sample after being struck by particles of the primary beam.
Arrows 14 and 16 show two exemplary paths that returning electrons may take. Those electrons taking path 14 strike detector 18. In the example shown the detector is an MCP or micro-channel plate detector and comprises a micro-channel plate but it may also be a pair of MCPs or a triple stack of MCPs 18a and an anode 18b. An MCP detector is shown because several of the problems given below are specific to MCP or MSP (micro-sphere plate) detectors. However the broad principle of the invention applies to any kind of detector that can be used in an electron microscope.
Electrons that follow path 16, however, do not strike the MCP and thus cannot be detected because a detector cannot be placed in the path of the primary beam. Furthermore any electrons that follow a path leading to dead area 20 cannot be detected either and the reason for this is as follows: The MCPs 18 must be held at a high voltage in order to attract particles and cause particle multiplication effects. Such voltages are sufficiently high to interfere with the primary beam 10 on its way to the sample and therefore the central passageway has to be shielded from the voltage. This is achieved by using a shielding tube 22, the walls of which are relatively thick. It would on the face of it be attractive to make the central passage long and thin, however a certain amount of interaction between the beam and the walls always occurs, resulting in deflection of the beam. Thus a minimum limit on the diameter of the central passageway of a significant fraction of the length of the tube is generally adhered to in order to minimize the interaction. As the length of the tube cannot be reduced below the thickness of the detectors 18 this sets a limit on how small the central passageway can be made.
In addition to the dead area caused by the central passageway itself that part of the detector immediately adjacent to the tube walls 22 cannot be charged to the voltage necessary to work, since it will cause a voltage breakdown at the wall 22, which is at a different potential. Therefore the dead area 20, in which no detection is possible, is relatively large and is particularly problematic in some applications in which important information is carried by those electrons that return from the sample in trajectories close to the axis of the microscope.
A further problem with the standard arrangement is that particles that return directly from the sample, which may include both primary and secondary electrons, occupy a range of energies, from those so weak that they cannot be detected to those so strong that they cause damage to the detector. The detector has certain energies within which detection efficiency is a maximum and not all electrons can be manipulation to lie simultaneously within that range.
In addition the MCP is exposed to being struck by pollutants, dust particles and the like, which can shorten the life span of the MCP by, for example, coating the active surfaces of the micro-channels so that particle multiplication is impeded.
A further difficulty is that the MCP 18 is limited by a relatively low saturation current. FIG. 2 shows a characteristic of input against output current for a series of different gains and it will be seen that no matter what the gain, an absolute saturation value of around 1.mu.A applies.